Traditional gas-phase spectroscopic absorption techniques use a laser, a gas cell or interaction region of some kind, and a photodetector to detect the intensity of light passing through the interaction region. Absorbed light at specific wavelengths indicates spectroscopic absorption features of a gaseous species. Photothermal spectroscopy is one conventional technique that employs a pulsed or modulated laser. The emission (beam) from the laser is absorbed by a gaseous analyte in a cell or interaction region that causes local heating within the gas. The heating of the gas changes the refractive index of the gas. The change in the refractive index is then detected (e.g., interferometrically) with a second laser. The second laser need not be the same laser as the first pulsed laser in either power or wavelength. Photoacoustic spectroscopy is another conventional technique in which the emission of a pulsed laser is absorbed by a gaseous analyte in a cell or interaction region that causes acoustic excitation of the surrounding gas. The acoustic excitation of the gas is detected with a microphone. In one variation, the laser emission is passed between the tines of a small turning fork and absorbed by gaseous analytes surrounding the tuning fork. Resulting pulsating pressure changes excite the fundamental mode of the turning fork, causing an electrical signal to be emitted through the electrical connections of the device which are subsequently detected.
Quantum Cascade Lasers (QCLs) are an important light source for chemical detection in the mid infrared (MIR) range (3 to 20 microns) because the emission wavelengths coincide with the fundamental absorption bands of many chemical species of interest. Tunable QCLs, in particular, external cavity QCLs (ECQCLs), are of particular appeal because they represent a single device with a typical tuning range that is 10%, and often up to 20%, of the center wavelength. Thus, only a few ECQCLs are needed to cover large swaths of the MIR. In conventional ECQCL configurations, the ECQCL is used to develop tunable wavelengths as optical outputs. The emission outputs interact with gaseous samples and allow their detection by various detectors (e.g., photodetectors) located external to the ECQCL. Both QCLs and ECQCLs (and other semiconductor lasers) can thus be used in concert with various traditional detection approaches.
However, in all of these approaches, despite the increasing capabilities of the lasers and their various laser configurations, the techniques are limited by the detection limits of the photodetectors and acoustic detectors in the MIR. The present invention addresses this problem by providing improved QCL and ECQCL laser configurations and methods for precise and sensitive detection of analytes without the need of a separate detector.